The Makedonian phalanx -- why such depth?

[PMBardunias]

Yes, varying the depth in this way is essentially a very simple system, which is almost universal. (Xenophon Cyropaedia II.3.21 for anyone who wants to look at it.) I have sent you this with explanation and diagrams, ....the meaning is so obvious... !

Obvious, yes, and I have defended your description of it for I believe it most likely correct. But the beauty of the system is that you need not double down all the way. This gives great flexibility. As you allude to above, based on the terminology for divisions the original formation must have been Lochos of 100, made up of 2 Pentekostyes of 50 that formed in 5 lines of ten- perhps doubling down to 10 of 5. Starting from a Lochos of 96 men, which is the number we get from Xenophon's dinner drill referenced above, we can form any number of ranks seen on a Greek battlefield. To do this you simply either interupt the doubling down process- stop at 8 instead of 4 for example. Alternately you divide by thirds and not halfs, as we know Spartans did to form 3's. An enomotia of 24, half of a Pentekontyes, can half to 12 ranks or third to 8 ranks. It can then double further, but the key is that it need not. Opened or close order is in no way fixed for each file length. For example a Pentekostyes of 48 can divide by 3 to give files of 16. These files of 16 can simply march up in close order if the officers wish.

It is the manner of division and not the unit size alone that governs the ending depth. To use the two examples from Xenophon, we see enomotia of 24 divide down through 12 to 6. While in Cilicia his real mercs divide down to 4. I see no reason to assume a different enomotia size for these men when simply dividing the enomotia of 24 into thirds give the depth of 4 by way of 8.

I attached an image of all of the different ways that Xenophon's basic drill can produce the known divisions of file depth. Including the early, perhaps ancestral split of 48 down to 8. You also see the 24 depth of the Thebans at Delium can be simply undivided enomotia in close order, while the famous 50 of Leuktra may just be undivided pentekostyes.

What we don't know is that any of this was done a century or two earlier. It might have been, but clan groups might just have formed behind a rank of leaders who line up alongside fellow clans.

there are a number of references to 'normal/open' and 'close' order in Xenophon - some of which I have referred to you in private correspondence long ago! e.g. against a Spartan attack along a road in column (Xen: VII.4.22" But the Arcadians stood firm.They formed up in close order and stood quietly...He (Spartan Archidamus) led his men up in double file, just as they were on the march, and so as the troops came to close quarters, the troops of Archidamus, marching, as they were along a road, were in column, and the Arcadians were in close order, shield to shield....."

The Spartans were in column marching in both examples- they had not deployed. If you assert that opened order was for marching along roads and troops deployed into close order when forming the battle-line, then we are in agreement.

At Mantinea (Thuc V.66 ff), we are told that the armies were already on the move in the final advance to contact …I would agree all units were most likely in close order. What is your point?

One of your main assertions for the existence of an opened order stage is that they had to advance in opened order due to the difficulty you perceive in moving in close order. Here at Mantinea they are obviously advancing the whole way in close order. There is no way that the phalanxes veered perhaps as much as hundreds of meters to the right in the course of a final 100 meter advance. From the description there is no reason to expect that they formed up in opened order, stood like that for a while, then doubled into close order. They formed directly from column into close order files and did not double down further. Then they simply advanced. Occam is surely on my side now.